Steel structures, such as power-transmission steel towers, are placed in environments in which they are exposed to the weather and susceptible to rust, and therefore anticorrosion treatment, such as galvanization, is carried out on the surfaces of the steel materials of such structures.
The surface of a steel base material that has been subjected to galvanization, such as hot dip galvanization, has a plating film usually comprising a δ1 layer and ζ layer both made of an iron-zinc alloy and formed in this order from the steel base material, and an η layer made of zinc and formed on the ζ layer. Galvanized steel structures have been said to be maintenance-free, and used without coating, or used with color coatings when they need to be distinguishable for the purpose of obstruction marking or the like, or when they need to be compatible with the surrounding environment.
The reality is, however, that the η layer of zinc is depleted more quickly than expected due to the effects of acid rain and the like in recent years, and there are many steel structures, such as power-transmission steel towers, in which the η layer has been eliminated to expose the ζ layer of an iron-zinc alloy, or the η layer and ζ layer have been eliminated to expose the δ1 layer, which is an iron-zinc alloy layer in contact with the steel base material. Red rust is gradually formed on steel structures in which the ζ layer or δ1 layer is exposed. Since red rust not only deteriorates the appearance but also becomes a factor in the reduction of the strength of such steel structures, it is necessary to provide anticorrosion coatings.
Japanese Unexamined Patent Publication No. 2000-140746 proposed a method for providing an anticorrosion coating on such a rusted steel structure, in which the surface of the steel structure is hand-cleaned and then coated with an undercoating, overcoating, etc.
The hand-cleaning is carried out according to the conditions of the steel structure as a substrate, and is usually performed using a power tool, such as a disc sander or the like, in combination with a hand tool, such as a scraper, hammer, or the like, to remove deteriorated coating films if any, and grind off the rusted portions until the metal surface is exposed. Insufficiency in such treatment of the substrate may become a factor in the reduction of the adhesion and anticorrosion performance of the anticorrosion coating film to be formed thereafter by applying an undercoating and overcoating, and may shorten the life of the anticorrosion coating film. However, when such an anticorrosion coating method is carried out on a power-transmission steel tower or the like, the method is performed in a high place without a scaffold and the manual cleaning process requires a considerably long time, causing the problems of prolonged power outage and great-safety and physical burdens on the workers. Recently, therefore, an anticorrosion coating is provided when the ζ or δ1 layer made of an alloy has been exposed but red rust has not been formed, but conventional undercoating compositions sometimes do not have sufficient adhesion to such exposed surfaces of alloy layers. Further, the cycle of recoating when using such a conventional coating method is ten and several years at the most. Under such a situation, it is difficult to sufficiently maintain and control the corrosion prevention of steel structures, such as power-transmission steel towers, which exist in considerable numbers.
To cope with these problems, Japanese Unexamined Patent Publication No. 2001-198521 proposed a coating method in which a one-component epoxy resin coating composition comprising an epoxy resin, a flaky pigment, a ketimine compound, etc., is used as an undercoating composition. This coating method is capable of forming a coating film with excellent adhesion and anticorrosion properties on the surface of a galvanized steel structure. However, at a portion where the η layer of zinc in the galvanization film has been depleted to expose the ζ or δ1 layer of an alloy, the coating film has insufficient adhesion and is unlikely to ensure long-term corrosion prevention, i.e., a recoating cycle of 50 years or more.